Lung infections cause the greatest burden of disease worldwide as well as the most deaths due to infectious disease in the United States. Understanding the mechanisms that control the innate immune response to lung infections is critical because an insufficient response exacerbates infection while an excessive response causes lung injury. Our long-term goals are to understand the mechanisms that regulate innate immunity in the lungs in order to rationally design therapies to control lung infections without causing unnecessary damage to the lungs. NF-KB plays a crucial role in the innate immune repsonse: it is a key regulator of cytokine expression. However, the precise role of different NF-KB proteins in regulating the innate immune response is not known and the role of these proteins in different cell types during different lung infections is not known. Based on our previous data using mice deficient in the NF-KB proteins RelA and on our studies of NF-KB protein functions in alveolar epithelial cells and macrophages, we propose the central hypothesis that the RelA subunit of NF-KB is essential in macrophages for host defense against Streptococcus pneumoniae pneumonia but not E. coli pneumonia. This hypothesis will be tested using the following specific aims: (1) Test the hypothesis that NF-KB RelA is necessary for alveolar macrophage expression of TNF-a, IL-la, and IL-1|3 stimulated by bacterial pathogens. (2) Test the hypothesis that NFKB RelA in macrophages is necessary to activate alveolar epithelial cells after exposure to S. pneumoniae but not to E. coli. (3) Test the hypothesis that NF-KB RelA in macrophages is essential for efficient host defense during S. pneumoniae pneumonia but not during E. coli pneumonia. The first goal will be achieved by measuring cytokine levels using quantitative RT-PCR and ELISA, RelA activation using scanning cytometry, and association of RelA with KB binding sites using chromatin immunoprecipitation. The second goal will be achieved using primary alveolar macrophages co-cultured with lung epithelial cells to measure cytokine expression as well as nuclear translocation of RelA in epithelial cells. The third goal will be achieved by infecting mice which lack RelA in alveolar macrophages (due to targeted Cre-mediated deletion) with either E. coli or S. pneumoniae and measuring the immune response (cytokine expression, bacterial clearance, neutrophil recruitment, and NF-KB activation). Together, these studies will allow us to determine the role of alveolar macrophage RelA during bacterial pneumonia and to use this knowledge to guide future research endeavors to develop therapeutics to treat lung infections by manipulating the innate immune response. By doing so, we could decrease the deadly and costly effects of pneumonia worldwide.